Raiders cornerback Nnamdi Asomugha has covered the opposing teams' top wide receiver more often this season than at any other point during his eight-year NFL career, yet opponents still find ways to exploit Oakland's defense whenever it employs a zone.

"There will be certain calls where he will (cover) another guy just because of the nature of the call and what we're doing," Raiders defensive coordinator John Marshall said.

When that happens, teams are quick to pounce.

Asomugha covered Michael Crabtree for most of the Raiders' 17-9 loss to the 49ers on Sunday and limited him to one reception for 10 yards. But with Asomugha not in coverage, Crabtree caught three passes for 47 yards, including a 32-yard touchdown that gave the 49ers a lead they never relinquished.

Crabtree's touchdown came on a post route against the zone. The player responsible for that part of the field was late reacting. Tyvon Branch and Chris Johnson are both taking the blame.

"Certainly after the play like the big post, you sit there and say, 'Man, I wish we were playing man there,' " Raiders coach Tom Cable said. "But that's an easy check, and it's an easy read. You just have to get to the post as a secondary player. It's not a difficult thing, but we didn't do it."

Against Arizona last month, Asomugha held receiver Larry Fitzgerald to one catch for 18 yards.

But Cardinals quarterback Derek Anderson connected with Fitzgerald for a touchdown on one of the few plays in which he wasn't covered by Asomugha. Cable said it didn't make sense to use Asomugha in man coverage on that play because the Raiders were in their red-zone defense, near the goal line.

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In a perfect Raiders world, Asomugha would be locked onto a team's top receiver throughout the game, regardless the situation, but that would entail too much preparation and take away from the overall scheme, Marshall said.

"You only have so much practice time," Marshall said. "So, if you keep him on one guy the whole time, that means he has to practice all those calls in various positions, and you just don't have enough snaps for it."

The goal is to use Asomugha on the team's top receiver for at least three quarters of the game and, certainly, on crucial downs.

Asomugha said it's impractical to play a mixed scheme where he is in man coverage while the others are in zone.

"In theory, it sounds like it works," Asomugha said of the mixed scheme, "but when we actually want to go out and execute it as a defense, there are times when it's not going to be about me locking on the guy. We're going to have to be able to play team defense in that zone scheme."

It doesn't work as well, Asomugha said, because the Raiders don't spend as much time practicing a zone concept as they do man coverage.

Asomugha likely won't shadow one receiver this Sunday against a Denver Broncos team with three wide receivers who have at least 32 receptions.

But you can bet Broncos quarterback Kyle Orton will know where Asomugha is every down.

"We're going to be real smart about what we do in terms of challenging (Asomugha) too many times," Broncos coach Josh McDaniels said, "because he's proved over the course of many, many years that that's not a very good formula."